It is known in the field relating to internal combustion engines to utilize cylinder pressure sensors for monitoring and controlling various processes, among them being combustion knock, misfire and associated combustion dilution and combustion phasing.
This invention relates to cylinder pressure sensors and more particularly to non-intrusive sensors having no direct contact with the harsh environment of a cylinder. Non-intrusive sensors measuring combustion pressure remote from direct contact with combustion are known which take various applications. Some applications include trapped force ring arrangements such as a piezoelectric annulus between cylinder head bolts and an engine block or alternatively between a spark plug seat and spark plug. Other non-intrusive sensors include probe type and annular insert type which respond to flexure of a first wall defining in part a cylinder relative to a second rigid wall in an engine head. The latter mentioned types of non-intrusive sensors, embodiments of which are set out in U.S. Pat. Nos. 4,601,196 to Frelund and 4,969,352 to Sellnau (both assigned to the assignee of the present invention), improve upon signal quality shortfalls of the former mentioned types and are preferred among alternatives.
Some advantages of the preferred probe type and annular insert type sensors over other non-intrusive pressure sensors include reduced dynamic phase lag, relative insensitivity to extraneous loads, lower operating temperature and other advantages due to their placement in proximity to the cylinder.
While the preferred non-intrusive sensors present substantial betterments to the field of cylinder pressure sensing, further improvements can be made. For example, a certain degree of non-linear response and gain variation from sensor to sensor are present with the preferred related art sensors. These undesirable characteristics tend to be exacerbated when the sensor structural material has a modulus of elasticity and yield strength of limited magnitude--such as is characteristic of aluminum and alloys thereof. This presents practical problems since modern combustion engines commonly have cylinder heads of aluminum alloys and, for reasons of matching thermal expansion characteristics of the sensor thereto, the sensor structural material is preferably similar.
Further, preferred non-intrusive sensors may tend to be sensitive to variations in installation preload caused for example by piezoelectric sensitivity, thread effects and other interface effects thereby demanding relatively tight control over preload. It is desirable that the preferred sensors be insensitive to installation variance such as is typically experienced when conventional torque monitoring is utilized to control threaded installations in an assembly environment, or alternatively as can be experienced in less controlled servicing situations.